1. Technical Field
The present invention relates generally to a piston structure including an oval skirt, and more particularly to an improved piston structure which has an oval skirt for uniformly absorbing a side thrust acting thereon during a power stroke.
2. Background Art
For reducing a piston weight, a so-called slipper type piston is well known in the art. This piston includes a slipper skirt having first and second thrust faces diametrically opposed to each other with respect to the centerline of the piston to contact with a cylinder wall during combustion. These thrust faces are supported by ribs extending from the pin boss portions.
However, in such prior art piston structures, portions adjacent both sides of the pin boss portions are cut out to provide two skirt segments, or first and second thrust faces, each having a relatively narrow circumferential width. Each thrust face is tapered in the axial direction of the piston in the form of an elliptical cone profile defined by a preselected major axis and minor axis with a constant difference therebetween ranging from an upper portion of the skirt to a lower portion thereof. It will be noted that side portions of the skirts have a relatively high rigidity. During combustion, side thrust loads concentrate on the highly rigid portions, thereby causing the skirt to seize. The narrower the circumferential widths of the skirts, the more the piston is subject to seizure. Prior art piston structures with narrow skirt widths for reducing piston weight do not ensure endurance against seizure or severe wear, or the like.